1. Field of the Invention
The present invention relates to a semiconductor device having a circuit comprising thin-film transistors on a substrate with an insulating surface and to a method of manufacture thereof. More specifically, the invention relates to a constitution of an electrooptical device represented by a liquid crystal display and of an electronic apparatus mounting the electrooptical device. In this specification, the semiconductor device generally denotes devices that function by utilizing semiconductor characteristics and includes the electrooptical devices and the electronic apparatus mounting the electrooptical devices.
2. Related Art
The thin-film transistor (hereinafter abbreviated to TFT) can be fabricated on a transparent glass substrate and therefore has been actively applied to active matrix liquid crystal displays. Because the TFT formed of a semiconductor layer having a crystal constitution (hereinafter referred to as a crystalline TFT) has a high mobility, high resolution image display can be realized by integrating functional circuits on the same substrate.
In this specification, the semiconductor film having the crystal constitution includes a single crystal semiconductor, a polycrystalline semiconductor, and a microcrystal semiconductor. It also includes semiconductors disclosed in official gazettes JP-A7-130652, JP-A8-78329, JP-A 10-135468, and JP-A 10-135469.
To construct an active matrix liquid crystal display, the pixel area alone requires 1 to 2 million crystalline TFTs and, when peripheral functional circuits are constitution, additional numbers of crystalline TFTs are necessary. To operate the liquid crystal display stably requires securing reliability of individual crystalline TFTs.
The characteristic of a field-effect transistor such as TFT can be constitution as comprising a linear region where a drain current increases in proportion to a drain voltage, a saturation region where the drain current saturates even if the drain voltage increases, and a cut-off region where the current ideally does not flow even when the drain voltage is applied. In this specification, the linear region and the saturation region are called ON regions of TFT and the cut-off region is called an OFF region. The drain current in the ON region is called an ON current and the current in the OFF region an OFF current.
The pixel area of the active matrix liquid crystal display comprises n-channel TFTs (herein after referred to as pixel TFTs) and, because a gate voltage with an amplitude of about 15-20 V is applied thereto, needs to satisfy both of the characteristics of the ON region and the OFF region. On the other hand, the peripheral circuits for driving the pixel area basically comprise CMOS circuits and their characteristics of the ON regions are mainly important. The crystalline TFTs, however, have a problem that the OFF current easily increases. Further, a degradation phenomenon has often been observed when driving the crystalline TFT for an extended period of time results in the mobility and ON current decrease and the OFF current increases. One of the possible causes is presumed to be a hot carrier injection phenomenon caused by a high electric field near the drain.
In the field of the MOS transistor, a lightly doped drain (LDD) constitution has been known as a method for reducing the OFF current and alleviating the high electric field near the drain. This constitution has a low concentration impurity region on the inner side of the source and drain regions, i.e., on the side of the channel forming region. This low concentration impurity region is called an LDD region.
The LDD constitution is also known to be formed similarly in the crystalline TFT. For example, with the gate electrode as a mask, a first impurity injection process forms a low concentration impurity region that will form an LDD region, and then sidewalls are formed on both sides of the gate electrode by using an anisotropic etching technique. With the gate electrode and the sidewalls as a mask, a second impurity injection process forms a high concentration impurity region that will form a source region and a drain region.
Although the LDD constitution can render the OFF current lower as compared with that of the ordinary constitution TFT, the LDD constitution has a drawback that because a series resistance component increases for the constitution reason, the TFT ON current, too, is reduced. Further, the deterioration of the ON current cannot be prevented completely. To compensate for this drawback a constitution has been known to be used in which the LDD region overlaps the gate electrode with the gate insulating film interposed therebetween. There are several methods for making this constitution, such as GOLD (Gate-drain Overlapped LDD) and LATID (Large-tilt-angle implanted drain). In this overlapping constitution the high electric field near the drain can be alleviated to increase the tolerance for hot carriers and at the same time prevent reduction in the ON current.
It is also confirmed that in the crystalline TFT, too, the provision of the LDD constitution increases withstandability against hot carriers as compared with that of the simple constitution TFT having only the source region, drain region and channel forming region, and that the use of the GOLD constitution produces an excellent effect. This fact is described in “A Novel Self-aligned Gate-overlapped LDD Poly-Si TFT with High Reliability and Performance,” by Mutsuko Hatano, Hajime Akimoto and Takeshi Sakai, TEDM97-523.
In the crystalline TFT, forming the LDD constitution has proved to be an effective means for suppressing the hot carrier injection phenomenon. Further, the use of the GOLD constitution is found to be able to prevent a reduction in the ON current, which was observed in the LDD constitution, and also to produce a preferable result in terms of reliability.
Although the GOLD constitution can prevent degradation of the ON current, it has a drawback that when a high gate voltage is applied thereto at the OFF characteristic, the OFF current is increased, as observed in the pixel TFT in particular. An increase in the OFF current in the pixel TFT produces undesired effects such as increased power consumption and abnormal image display. This is constitution due to an inverted layer which, because of the characteristic of the OFF region, is formed in the LDD region overlapping the gate electrode.
As described above, to realize a high level of reliability in the crystalline TFT requires a comprehensive study on the constitution of the device. For this reason, it has been desired that the GOLD constitution be formed. With the conventional method, however, although the LDD region can be formed self-aligningly, the process of forming the sidewall film by an anisotropic etching is not suited for processing a large area glass substrate like the one used in the liquid crystal display, Further, because the length of the LDD region is determined by the width of the sidewall, the degree of freedom in designing device dimensions is severely restricted.
If, as in the pixel TFT, the characteristics of both the ON region and the OFF region are important and an attempt is made to meet the required level of their reliability and prevent an increase in the OFF current, the conventional GOLD constitution is not sufficient.